The invention relates to a method and an apparatus for the purification of a hazardous residual product from the process carried out in a flue gas purification plant for acid gas purification to obtain a waste dump-safe residual product by use of water, preferably waste water from wet-desulphurization plants. The water is subjected to aftertreatment in a waste water-treatment plant.
Environmental requirements that combustion smoke from industrial plants, e.g. waste combustion plants, be purified of acid components before the smoke is released into the atmosphere, are now so widespread that flue gas purification plants which comply with such requirements are commonly used in many countries.
The most widely used state of the art plants are based on the use of calcium as acid neutralizer, calcium in the form of a wet or a dry reactant being added to the flue gas in the plant. The calcium reacts with the flue gas to form reaction products in the form of soluble salts, such as CaCl.sub.2, and non-soluble salts, such as CaSO.sub.3 and CaSO.sub.4, which are subsequently removed from the plant as a residual product in the form of a sludge or a powder. At the same time the desulphurized flue gas is released into the atmosphere.
Such flue gas purification plants may be divided into three main categories, as disclosed in e.g. U.S. Pat. No. 4197278, viz.
wet-process plants in which the flue gas is `scrubbed` in an aqueous suspension or solution of calcium and which produces a harmless sludge-like residual product, CaSO.sub.4.2H.sub.2 O (wet gypsum) and waste water, PA1 dry-process plants in which the flue gas is treated with a dry absorbing agent (calcium) and from which the reaction products are extracted as a hazardous, dry powder containing HCl and HF, and PA1 semi-dry-process plants in which the reactant is supplied in the form of an aqueous solution, the water, however, being caused to evaporate during the process so as to extract, in this plant too, the residual product in the form of a hazardous, dry powder.
So far wet-process plants have been preferred for flue gas purification because the most effective desulphurization of the treated flue gas is obtained therein with limited use of calcium, also in case of a high SO.sub.x -content. The inconveniences associated with the wet-method are the formation of a wet residual product, the use of large amounts of fresh water for the `scrubbing` proper in the plant, and the formation of an amount of waste water which is to be aftertreated for the sake of the environment.
In dry-process- and semidry-process plants the SO.sub.x -removal from the flue gas is less effective than in wet-process plants. On the other hand there is no waste water problem associated with such plants and the residual product, dry powder, is easier to handle, whereas it is an inconvenience that the soluble salts formed in the processes of such plants by deposition of residual products are leached out so that depositing can only be performed on particularly suitable locations.
The large consumption of water in wet-process plants is due to the technical requirement that the salts removed from the flue gas should be washed away for final discharge into streams or sea areas. However, the salt concentration proper of the waste water produced is considerably lower than required with due regard to the recipient, e.g. a sea area. Prior to discharge, the waste water should receive final treatment in a waste water-treatment plant coupled after the flue gas purification plant. Among other things, the acid-content of the water should be neutralized, e.g. by addition of calcium during the treatment process.
Thus, with its salt subsaturation and a low pH prior to its treatment in the waste-water plant, the waste water represents per se a resource which has not been utilized so far and which may be rendered useful for the conditioning of waste dump-hazardous residual products from flue gas purification plants into waste dump-safe residual products, as the soluble salts and the leachable heavy metals are leached out of the hazardous residual products by means of the waste water.
DE-A-3320466 discloses a method of cleaning flue gas where two subsequent steps are used for cleaning the same flue gas stream. However, this prior art publication does not suggest the cleaning of a residual product from one process by use of the residual product from another process.